1. Field of the Invention
This patent relates to automatic control devices. More particularly, this patent relates to electric switches for controlling and monitoring the pressure in a system.
2. Description of the Related Art
Pressure switches are used to regulate the pressure within a system. For example, pressure switches can be used to regulate the air pressure within a tank used to supply air for spray painting. Standard pressure switches have a diaphragm which is open to the tank allowing the pressure switch to sense the pressure in the tank. The pressure drops as the air in the tank is used up. When the pressure in the tank reaches a minimum acceptable level, the pressure switch causes electrical contacts to close which starts up an electrical motor. The electrical motor runs a compressor which refills the tank with ambient air.
Pressure switches can operate a motor directly or operate a magnetic motor starter. In the latter case, the pressure switch acts as a relay to turn the starter on and off.
When a mechanical problem (e.g., a bad bearing) or an electrical problem (e.g., a low voltage condition) occurs in a compressor motor, the current passing through the motor increases to compensate, thus causing the motor to work harder. As the motor works harder, the motor temperature increases. When the temperature reaches the thermal limit of the insulation surrounding the motor, the motor can burn out, causing a short.
A significant disadvantage of most conventional pressure switches, such as those disclosed in Bauer U.S. Pat. No. 3,949,179 and Willcox U.S. Pat. No. 3,875,358, is that they do not have a thermal release device. That is, they have no means of sensing increased electrical current through the compressor motor and automatically shutting off if the current exceeds an acceptable level (i.e. overamperage).
Switches that have a thermal release device, such as Henning U.S. Pat. No. 1,841,477, have the disadvantage that, due to their one piece construction, they cannot be easily modified to match the amperage of a motor.
Another disadvantage of conventional pressure switches is that they cannot be turned off by remote control via an undervoltage release or shunt trip. Such remote control devices allow the compressor to be turned off by remote control in the case of an emergency.
Thus, there is a need for a pressure switch having a thermal release device and a remote control tripping device integrated into a single unit. The present invention provides such a device in a compact, modular design.